Practical activity - paper chromatography of ink
There are several ways that you could investigate the composition of inks. This is an outline of the required steps to undertake one of these methods.
Aim
To identify the mixture of dyes in a sample of unknown composition.
Method
- Draw a pencil line across the Chromatography is used to separate different substances dissolved in a liquid. paper, 1 to 2 cm from the bottom
- use a pipette or capillary tube to add a small spot of the unknown ink to the line on the paper
- add dot of a range of dyes to the paper
- place the paper into a container with a suitable The liquid in which the solute dissolves to form a solution. in the bottom
- allow the solvent to move through the paper, but remove the The results of separating mixtures by chromatography. before the solvent reaches the top
- mark the distance that the solvent has travelled
- then measure the distance travelled by each spot and by the solvent
Results
Record your results in a suitable table. For example:
| Ink | Spot colour | Distance travelled by spot (mm) |
| Ink | |
|---|---|
| Spot colour | |
| Distance travelled by spot (mm) |
| Ink | |
|---|---|
| Spot colour | |
| Distance travelled by spot (mm) |
| Ink | |
|---|---|
| Spot colour | |
| Distance travelled by spot (mm) |
Analysis
- calculate the Rf value of each spot: \(Rf = \frac{distance\ travelled\ by\ substance}{distance\ travelled\ by\ solvent}\)
- compare the Rf values and colours of each spot in the inks and describe their similarities and differences
- compare the Rf values of the spots from the unknown ink with the Rf values of the dyes
Evaluation
Question
Explain why the distances travelled by each spot were measured in mm, rather than in cm.
Measurements in millimetres are more precise than those in centimetres. They will give more precise Rf values.
Hazards, risks and precautions
It is important in this practical activity to use appropriate apparatus and methods. This includes the safe use and careful handling of substances. Eye protection must be worn.
Evaluate the hazards and the precautions needed to reduce the risk of harm. For example:
| Hazard | Possible harm | Possible precaution |
| Harmful solvent | Skin irritation | Avoid skin contact, eg wear gloves |
| Harmful solvent | Breathing difficulties | Ensure adequate ventilation or use a fume cupboard |
| Hazard | Harmful solvent |
|---|---|
| Possible harm | Skin irritation |
| Possible precaution | Avoid skin contact, eg wear gloves |
| Hazard | Harmful solvent |
|---|---|
| Possible harm | Breathing difficulties |
| Possible precaution | Ensure adequate ventilation or use a fume cupboard |
